1. Field of the Invention
This invention is applicable to measurements made on optical communication equipment. More particularly, this invention is applicable to measurement of modulation characteristics of optical signals which have been modulated in frequency. This invention device is most suitable to measure modulation characteristics of optical signals for coherent light communications which have been emitted from a laser diode and modulated in frequency.
2. Prior Art of the Invention
Frequency modulation has been known as an effective method for coherent optical communications. Simple transmitters can be made by using frequency-modulated signals obtained by inputting modulation signals directly to the laser diode. Frequency-modulated signals obtained by directly modulating the laser diode, however, contain an amplitude modulation component as well as a frequency modulation component. Accurate measurement of the frequency modulation component requires a measurement method which is relatively independent of the influence of the spurious amplitude modulation component.
The frequency modulation response of the laser diode is influenced mainly by the frequency changes caused by the thermal effect at a low modulation frequency region, and mainly by the effect of the carrier in the high modulation region. The direction of frequency changes of these two effects, however, are opposite in sense to each other, and the frequency modulation response to injection current of the laser diode is accordingly not uniform. In order to measure the frequency modulation response of the laser diode, a circuit is required which can faithfully convert changes in frequency of optical signals into changes in voltage.
A Fabry-Perot etalon has conventionally been utilized for measurements of this type. A Fabry-Perot etalon is a device comprising two parallel plates, between which an object light is entered at an incident angle. An interference pattern is formed when the light is repeatedly reflected between the two parallel plates. If the distance d between the two plates and the angle .theta. are fixed, the interference pattern is determinable by the frequency of the incident light. Etalons which measure changes in frequency (or wavelength) of light by the interference pattern are widely known.
FIG. 5 is a characteristic graph which shows the relation between the input light frequency and the output light amplitude of such an etalon. The optical frequency is plotted on the horizontal axis while the optical power of the output light is plotted on the vertical axis. In other words, the graph shows a relationship between changes in intensity of the light which is outputted from the etalon, against frequency changes. If a point a at a large gradient of the characteristic curve is selected in the graph, and an incident light which has been modulated in frequency, with a center frequency fa of the point a, is given to an etalon, the optical signals in terms of changes in optical power are obtained as shown by the curve b in FIG. 5. The frequency modulation response can be measured by converting to voltage signals, with a photodetector, the optical signals which have already been converted to optical power change.
However, the above-mentioned instrumentation method has the following problems:
(1) As steep changes in frequency are limited, it cannot measure a wide frequency range;
(2) as it uses multiple reflections, propagation time difference between reflected waves and traveling waves prevents the measurement of frequency modulation response in a high frequency range; and
(3) distortion occurs when the frequency deviation of input light is large.
Particularly, as to the first problem, a technique is known of varying the measurable frequency range by moving one of the reflective mirrors of the etalon or by changing the incident angle of the light, but the method needs an extremely precise and complicated mechanism.
Another method which conceivably causes interference patterns is the use of Mach-Zender interferometers. Although the Mach-Zender interferometer has an excellent response characteristic even in the high frequency range and shows gentle changes in optical power over a wider range of frequency of the input light, the light signals which have passed through a Mach-Zender interferometer are directly influenced by the amplitude changes of the input light. Therefore, the Mach-Zender interferometer has been regarded heretofore as not being suitable as an instrumentation device for separately measuring the frequency modulation component and the amplitude modulation component.
This invention was conceived in order to solve such problems encountered in the prior art and aims to provide an instrumentation device which can measure frequency modulation component over a wide frequency range and which has an extremely simple mechanism.